Three basic processes have been developed for the gasification of carbonaceous materials such as coal. They are: (1) fixed-bed gasification, (2) fluidized-bed gasification, and (3) suspension or entrainment gasification. The present invention relates to the third type of process, suspension or entrainment gasification.
One method of carrying out an entrainment gasification process to produce synthesis gas, hereafter "syngas", is described in U.S. Pat. No. 4,872,886 to Henley. In that process, a two-stage gasification reaction in a non-catalytic, fired horizontal slagging reaction zone, or first stage reactor, a stream of oxygen-containing gas is reacted with a first increment of a slurry of particulate carbonaceous solids in a liquid carrier at temperatures from 2400.degree. F. (1316.degree. C.) to 3000.degree. F. (1649.degree. C.). The oxygen, carbonaceous solids and liquid carrier are converted into steam, vapor from the liquid carrier, char, slag and gaseous combustion products. The slag which forms in the reactor flows by gravity to the bottom of the reactor and out of the reactor through a tap hole. In a second stage, the steam, vapor from the liquid carrier, char and gaseous products from the fired horizontal reactor are contacted, in an unfired vertical second stage reactor, with a second increment of slurry of particulate carbonaceous solids in a liquid carrier to yield steam, vapor from the liquid carrier, syngas and char entrained in the gaseous effluent. In addition, small sticky slag droplets are entrained and will tend to adhere to and foul surfaces which they contact. The temperature at which the slag droplets solidify ranges over a wide band of temperatures usually upwards of about 1700.degree. F. (927.degree. C.). During this sticky phase these molten slag particles or droplets exhibit sufficient stickiness that they can cause extreme difficulties in processing because the particles or droplets adhere to and form deposits on walls, valves, outlets, and the like of process equipment downstream of the gasifier. Usually, one such piece of downstream equipment is a high temperature heat recovery unit or boiler, which in the case of Henley U.S. Pat. No. 4,872,886 is a fire-tube boiler. The fire-tube boiler has the product dirty syngas flow through the tubes and the heat exchange fluid, usually water, flows on the shell side. While it is considerably more economical to use a fire-tube boiler from the standpoint of capital expense, the plugging of the tubes creates a disadvantage of increased maintenance expense. Because the feed coal slurry is finely divided, any char particles and the slag droplets would normally pass through the tubes of a fire-tube boiler. However, the agglomeration of particles within the passageways of the reactor gaseous exit piping, the sloughing off of deposits which have previously built up on the walls and other points of flow interruption within the reactor passageways and the coating and build up of deposits in the entrance of the boiler, all cause the narrowing of the boiler tube openings which increases pressure drop across the boiler and eventually plugs the boiler tubes. It is therefore desirable to provide a system or apparatus which will prevent the plugging of the boiler or at least delay such plugging or pressure drop increase for as long as possible prior to having to shut down the gasifier and clean out the boiler tubes.
Various filters have been attempted but have proven unsatisfactory because they either blind themselves too quickly or when partially blinded can not withstand sufficient pressure drop themselves without collapsing. It has now been found that a combination of various filter screens allows small particles of char and slag to pass through the tubes of a fire-tube boiler without plugging while still preventing the larger particles of char-slag agglomerate or sloughed off deposits to be caught and held away from the entrance to the fire-tube boiler tubes and still allow the dirty syngas to pass through the screen combination at reasonable pressure drops.
This combination of advantages has been found to provide satisfactory use in a coal gasification process which has operated under commercial conditions, allowing continued operation for reasonable periods of time. These and other objects and advantages are accomplished in accordance with the present invention as described hereinbelow.